This invention relates to debris caps for subsea wellheads and xmas trees.
In the exploitation of offshore oilfields, after a well is drilled, it may be left for several months, or even years, before it is completed and placed into production mode (xe2x80x9ctemporary abandonmentxe2x80x9d). Similarly, a subsea xmas tree may not be used for production immediately after it has been installed at the wellhead. In such circumstances there is a need to protect upper external and internal wellhead and subsea xmas tree sealing surfaces, locking profiles and other vulnerable components against corrosion. Protection against damage or obstruction by falling debris, silt and biological accretions is also required.
For these purposes a debris cap will be installed on the upper end of the wellhead housing or xmas tree. It is deployed by wireline using a special running tool. A ROV actuated lockdown mechanism is used to secure the cap in place. The ROV is also used to deliver, to the volume of stagnant seawater trapped beneath the cap, via an umbilical connection, a solution containing corrosion inhibitors and biocides. This provides corrosion protection for the various types of materials within the stagnant volume as required (for example AISI 8630 steels, Inconel(copyright) 718 and 625 alloys, elastomers such as HNBR, PTFE or plastics such as Devlon(copyright) V-API). The biocide will also inhibit bacterial growth.
Feedback from customers and end users has highlighted certain flaws with the existing design of debris caps. Their installation procedures are relatively complex and time consuming, involving use of the wireline running tool, followed by ROV manipulation of the lockdown mechanism and connection/disconnection of the corrosion inhibitor delivery umbilical. The complexity can lead to unexpected installation difficulties, still further increasing the required ROV deployment time.
It is an objective of the invention to eliminate the use of an umbilical line or an ROV to pump corrosion inhibitor into the stagnant volume beneath a debris cap. Accordingly the present invention provides a debris cap for a subsea xmas tree or wellhead, comprising container that can be pre-charged with corrosion inhibitor and/or biocide prior to installation subsea, and which releases the corrosion inhibitor/biocide into a stagnant volume enclosed beneath the cap following installation. Preferably the debris cap can be run on wireline alone or deployed by an ROV alone, advantageously with no need for a special running tool or any lockdown mechanisms. This will result in installation time and ROV deployment time being shorter. The preferred debris cap of the invention may also be diver installable without the use of special tools.
For a fuller understanding of the invention and its preferred features, illustrative embodiments are described below with reference to the drawings.